function plot_pgram(p0, v1, v2)

plot([p0(1), p0(1)+v1(1)], [p0(2), p0(2)+v1(2)]);  
hold on;      
plot([p0(1)+v1(1),  p0(1)+v1(1)+v2(1)], [p0(2)+v1(2), p0(2)+v1(2)+v2(2)]);
plot([p0(1), p0(1)+v2(1)], [p0(2), p0(2)+v2(2)]);
plot([p0(1)+v2(1), p0(1)+v2(1)+v1(1)], [p0(2)+v2(2), p0(2)+v2(2)+v1(2)]);

